The S1P3 receptor gene encodes for a member of the endothelial differentiation gene (EDG) family of receptors widely present in central and peripheral human tissues (Rosen et al., 2009; Ishii et al., 2001). S1P3 receptor (also called: EDG3; LPB3; S1PR3; EDG-3) belong to a class of five (S1P1-5) seven-spanning membrane proteins belonging to the class of G-Protein-Coupled Receptors (GPCRs), whose natural ligand is the bioactive lipid sphingosine-1-phosphate (S1P) (Chun et al., 2002). S1P is involved in a large array of cellular responses modulating several physiological processes such as innate immunity, wound healing, vascular endothelial cell functions, inflammatory response and others (Ishii et al., 2004; Brinkmann, 2007; Rosen et al., 2009; Maceyka et al., 2012). S1P is intracellularly produced, with the direct role of secondary messenger (Spiegel and Milstien, 2003), and extracellularly exported acting to S1P cell membrane receptors as endogenous ligand.
The S1P receptors expressed in many apparatuses are able to trigger signalling through a variety of heterotrimeric G proteins, including Gi/o, G12/13, and Gq. In the S1P1-5 receptors family, S1P3 has been shown to be functionally relevant in several physiological processes such as regulations of heart rate, angiogenesis and vascular contraction (Forrest et al., 2004; Sanna et al., 2004; Marsolais and Rosen, 2009; Means and Brown, 2009; Murakami et al., 2010), in embryonic angiogenesis development (Kono et al., 2004) or as autophagy modulator (Taniguchi et al., 2012). The S1P3 receptor is also deeply involved in immunological processes (Brinkmann V. (2009). Importantly, mice lacking S1P3 receptor did not show evident abnormalities indicating a non-essential role of the receptor for a normal animal development (Ishii et al., 2001). As mentioned, S1P plays an important role as essential modulator of innate immunity and inflammation inducer. S1P once produced and released as signalling molecule by a wide range of cell types or even non-nucleated cells (e.g. platelets) (Pyne and Pyne, 2000) can exert an important role in inflammation. As introduced, together with the whole S1P receptor family, S1P3 receptor system has been largely studied focusing on its role in disease and has been shown to be involved in a large number of pathologies. From the literature S1P3 emerges as an important target implicated in pathologies with inflammatory components, in this case a pharmacological inhibition of the receptor could potentially counteract the disease evolution. The S1P3 receptor appears to be an appealing target also for other therapeutic areas, in which a potential healing role of S1P3 antagonism has been demonstrated.
S1P3 Antagonism in Peripheral Diseases
S1P3 activity has been shown to be implicated in inflammation-associated diseases such as arthritis (Lai et al., 2010), and several type of fibrosis (Shea and Tager, 2012) like heart (Takuwa et al., 2010), pulmonary (Kono et al., 2007), muscular (Cencetti et al., 2008) and liver fibrosis (Li et al., 2009) or in other more general inflammatory syndromes (Niessen et al., 2008) where S1P3 receptor antagonism could potentially limit the pathologic processes.
S1P3 receptor activation in the cardiovascular system could exert several pathologically-relevant effects. In the blood the S1P released by activated platelets stimulates S1P3 (and S1P1) receptors in vascular endothelium decreasing vascular para-cellular permeability (Mehta et al. 2005; Sun et al. 2009). Additionally, S1P3 transactivation has been shown to disrupt vascular barrier regulation (Singleton et al., 2006). Furthermore, also the chemotactic effect of S1P in macrophages (demonstrated in vitro and in vivo) is mediated by S1P3, so playing a causal role in atherosclerosis by promoting the recruitment of inflammatory monocyte/macrophage and altering vessel smooth muscle cells behaviour (Keul et al., 2011). Finally, the group of Takakura has demonstrated by a specific antagonist that the S1P-induced coronary flow decrease is dependent on S1P3 receptor and so such antagonism might be adapt to counteract S1P related vascular diseases and vasospasm syndromes (Murakami et al., 2010). In the heart, interestingly, the sustained bradycardia induced by S1P receptor non-selective agonists is abolished in S1P3 knockout mice or after S1P3 pharmacological inhibition in rats (Sanna et al., 2004; Murakami et al., 2010). More, in the cardiac vascular microcirculation cells in diabetes, it has been shown in vivo and in vitro that the agonist FTY720 exerts a functional antagonism by stimulating the translocation of S1P3 from membrane to the nucleus. Arguably, the pharmacological modulation of S1P3 receptors could be beneficial to alleviate cardiac microangiopathy in diabetes (Yin et al., 2012).
Bajwa et al. (2012) have demonstrated that S1P plays a pivotal role in kidney ischemia-reperfusion injury (IRI). S1P3 receptor-deficient mice were protected from IRI. This protective effect was due, at least in part to differences between S1P3-deficient dendritic cells. It was then supposed that pharmacological treatment are able to limit S1P3 activity or treatments with dendritic cells lacking the S1P3 receptor could help against progression of IRI.
Also several physiological parameters of the respiratory system are affected by S1P3 activity. It has been recently demonstrated that the S1P pathway activation induced a generalized airway hyperreactivity in vivo and in vitro and this is mediated by S1P3 receptor. Then, the S1P3 antagonism, besides or contextually to the abovementioned putative healing effects on lung fibrosis, could represent a new therapeutic strategy aimed at blocking the asthma-related airway hyperreactivity (Trifilieff and Fozard, 2012). S1P3 has been also shown to be strictly involved in acute lung injury where it promotes chemotaxis and increased endothelial and epithelial permeability (Uhlig and Yang, 2013). In the publication of Chen et al., (2008) it is suggested that S1P acting through S1P3, increasing calcium influx, and Rho kinase, activates cPLA(2)alpha and releases arachidonic acid in lung epithelial cells. Then, understanding this mechanism in epithelial cells may provide potential targets to control inflammatory processes in the lung.
S1P3 receptors play an important role in other non-inflammatory diseases. In cancer, it has been shown that S1P3 activation promotes breast cancer cells invasiveness (Kim et al., 2011) and this effect can be diminished by a specific antibody able to block the receptor (Harris et al., 2012). Similar results were obtained in thyroid cancer cells (Balthasar et al., 2006) and glioma cells (Young et al., 2007), where S1P3 activation showed to enhance cell migration and invasion. Yamashita (2006) also demonstrated that S1P3-mediated signals might be crucial in determining the metastatic response of gastric cancer cells to S1P.
In the eye, considering that S1P is constitutively present in the aqueous humor (Liliom et al., 1998), and, in addition, that the endothelial cells of the trabecular meshwork, which express S1P1 and S1P3 receptors (Mettu et al., 2004), respond to S1P stimulus increasing the outflow resistance, the S1P3 receptors pharmacological inhibition represents a potential therapeutic strategy in healing pathologies involving high intraocular pressure such as ocular hypertension, glaucoma, glaucomatous retinopathy (Stamer et al., 2009).
S1P3 antagonism in PNS diseases
The tissue injury inflammation is associated with an increased sensitivity to noxious stimuli, suggesting that there could be an important interaction between the activities of immune cells and the sensory neurons activated by noxious stimulation. A direct exposure of isolated sensory neurons to S1P (together with other inflammatory signals released by platelets or mast cells) increases their action potential firing through activation of ion channels (Zhang et al., 2006). In experimental conditions of isolated sensory neurons, the expression of S1P3 receptors is the highest in the panel of S1P receptors. In addition, the Kress's laboratory has demonstrated that S1P3 receptor was detected in all human and mouse dorsal root ganglia neurons and that S1P evokes significant nociception via G-protein-dependent activation of an excitatory chloride conductance (Camprubi-Robles et al., 2013). Considering that S1P-induced neuronal responses and spontaneous pain behavior in vivo were strongly reduced in S1P3-null mice, S1P3 receptors could represent important therapeutic targets for post-traumatic pain (Camprubi-Robles et al., 2013).
S1P3 Antagonism in CNS Diseases
In the CNS, neurons, astrocytes, oligodendrocytes and microglia cells have the capacity to produce and secrete S1P and express, with different extents depending on the cell type, S1P1-3 and S1P5 receptors (Anelli et al., 2005; Foster et al., 2007). In regard to S1P3 receptor, an intrinsic high expression has been seen in both astrocytes and neurons (Foster et al., 2007). S1P3 is described to induce glial activation under pro-inflammatory conditions (Fisher et al., 2011; Wu et al., 2008) and enhance spontaneous glutamate release in the hippocampus mossy fibers (Kanno and Nishizaki, 2011). In particular, apoptotic neurons self-induce an overexpression of sphingosine-kinase with a further release of S1P. This process, elegantly demonstrated by Gude (Gude et al., 2008) and defined as “come-and-get-me” signal, has the purpose of chemo-attract microglial cells and eliminate the dying neuron. Furthermore, S1P through a G12/13 protein, by remodelling of actin cytoskeleton, can inhibit astrocytes tight junction, conferring them mobility, and creating gaps through the brain tissue (Rouach et al., 2006). Then, the action of S1P to astrocytes could help the activated microglial cells to better move in the brain and so express their phagocytic role. The S1P3 receptors coupling to the G12/13 protein, associated to a high S1P3 receptor expression in astrocytes and its role in motility (Fischer et al., 2011) leaded to the consideration that the described process could be conducted by a S1P3-activated signalling. Interestingly, microglial cells, once activated, enhance their S1P3 expression (Foster et al., 2007). With these evidences it was conceivably hypothesised that activation of S1P3 receptor system is strictly involved in a neuroinflammatory state and S1P3 inhibition could have limited its development. Evidence supporting S1P3 antagonism to be protective in neuroinflammation was given from a mouse model of Sandhoff disease in which the ablation of the gene encoding S1P3 receptor strongly limited the astroglial proliferation, prolonging the survival and improving motor function of the mice (Wu et al., 2008).
In neurodegenerative diseases neuroinflammation can play a clear detrimental role during the pathologic evolution Alzheimer's, Parkinson's, Amyotrophic lateral sclerosis, Huntington and Multiple sclerosis (Bradl and Hohlfeld, 2003; Maragakis and Rothstein, 2006; Davies et al., 2013). In Alzheimer disease (AD) the accumulation of beta-amyloid plaques has been associated to inflammation development with activation of the CNS immune system (Meraz-Rios et al., 2013). A relevance of S1P receptors activity and modulation in AD is also shown in Takasugi et al., 2011 and in Takasugi et al., 2013